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the variable speed motor in the furnace rotates a fan that supplys the air that circulates through your ducts so If you have a variable speed furnace you will have a variable speed air conditioner---you can use a 2 stage compressor in the condensor as well

if you are having a home built you should find an HVAC contractor that you can discuss your needs with you . he will need to review your plans to determine the proper size units you will need

YOu really need to hire a contractor that can produce a Manual J & D on the house. Let him guide you on what system will meet your specific requirements. Generally speaking, Variable Speed Blowers and Compressors (AC) come in two types: Multispeed and Variable Frequency Drives.

Most common is Multispeed blower and compressors which have a high and low speed, and sometimes a medium speed.

More expensive and less common is VFD motors which actually ramp up based on demand from the thermostat and/or control board.

What I tell my customers is that a low end, cheap system designed and installed to a high degree of excellence outperforms a great system with the best technology available, installed to the typical standards of your average lowbid contractor.

So, you're asking the wrong question. What you should be asking is who is the best contractor in my area, what are his credentials and see that he installs your system, regardless of whatever equipment you choose to the best possible standards.

All a variable speed furnace does is guarantee( to a certain degree) that you will move a certain CFM of air. They tend to be quieter also and more energy efficient. You don't need a variable condensor, or even a 2 stage a/c. However if you have the money a 2 stage a/c is nice. I don't think I've ever heard of "variable speed a/c" 2 is about all you get.